The present invention relates to deoxidation casting, aluminum casting and a casting machine.
Many ways of casting, e.g., gravity casting, low pressure casting, die casting, squeeze casting, thixotropic casting, are known. In many ways of casting, a molten metal is poured into a cavity of a casting die to solidify and form the metal into a prescribed shape. The ways of casting are selected on the basis of a material of the molten metal and a product to be cast.
Many kinds of products are cast. In the case of casting a product having a complex shape or high performance, the cavity must be securely filled with the molten metal so as to form no casting defects, have a prescribed strength, prevent deformation, and have a good external shape.
Aluminum and aluminum alloys have been widely used as the material of the molten metal. In the aluminum casting, aluminum is apt to make an oxide film. Due to the oxide film formed on a surface of the molten aluminum, the surface tension of the molten metal is made greater, so that fluidity and a welding property of the molten metal is made lower and casting defects are sometimes caused. To solve these disadvantages, many improvements, e.g., lubricant, pouring manners, pouring speed, pouring pressure, have been studied.
For example, in gravity casting and low pressure casting methods, the falling temperature of the molten metal is slowed by painting heat-insulating releasing agent, adjusting an arrangement of a gate, etc., so that bad molten metal running, crinkles, cold shuts, etc., which are caused by forming the oxide film on the surface of the molten metal, can be restricted. In die casting, the molten metal is filled in a short time with high pressure by adjusting the pouring speed, the pouring pressure, the arrangement of the gate, etc. In squeeze casting, the pouring pressure is highly pressurized during a step of the gravity casting so as to break and fuse the oxide film.
However, the conventional ways of casting have disadvantages, and no perfect ways are known. Especially, the oxide film, which is made or formed when the molten metal touches the inner faces of the cavity of the casting die, forms crinkles and cold shuts in a surface of a product, and the oxide film causes unsatisfactory filling of the molten metal. In the case of casting parts of airplanes and vehicles, whose surface stress and broken portions seriously influence safety, etc., all cast products are examined by means of fluorescent flaw detection. Therefore, the manufacturing cost of the products is higher. Further, the quality and reliability of the products cannot be fixed.
The problems of the oxide film are caused not only in aluminum casting but also casting with other materials.
The present invention has been invented to solve the problems caused by the oxide film formed on the surface of the molten metal.
An object of the present invention is to provide a method of deoxidation casting, which is capable of preventing the oxide film from forming on the surface of the molten metal, improving wettability to the inner faces of a cavity of a casting die, and casting high quality products with high casting efficiency.
Another object is to provide a deoxidation casting machine for executing said method.
The method of deoxidation casting of the present invention comprises the steps of:
reacting a deoxidizing compound, which is made by reacting a metallic gas on a reactive gas, on a molten metal; and
deoxidizing an oxide film on a surface of the molten metal.
A metal for the metallic gas may be selected on the basis of the molten metal. For example, magnesium nitride compound (Mg3N2) which is made by reacting a magnesium gas with a nitrogen gas, may be employed as an effective deoxidizing compound, which is capable of deoxidizing the oxide film formed on the surface of the molten metal. Magnesium is stable from room temperature to high temperature and capable of easily subliming. Therefore, magnesium can be properly used in the method. The magnesium nitride compound has a high deoxidizing property, so that the oxide film on the surface of the molten metal can be effectively deoxidized.
The deoxidation casting of the present invention relates to a method which is capable of deoxidizing the oxide film formed on the surface of the molten metal so as to make the pure molten metal. Therefore, in the case of casting with a molten metal on which the oxide film is apt to be formed, the method of the present invention is capable of effectively deoxidizing the oxide film and properly casting with the pure molten metal.
By deoxidizing the oxide film on the surface of the molten metal, the surface tension of the molten metal can be lower, fluidity of the molten metal can be higher and the wettability with respect to the inner faces of the cavity of the casting die can be higher. Since the pure molten metal touches the inner faces of the cavity, the molten metal can easily flow in the casting die, the molten metal running property can be improved and the molten metal can securely fill the cavity including minute spaces thereof.
In a conventional casting method, lubricant or heat-insulating releasing agent are used to warm a casting die and maintain the fluidity of a molten metal. In the present invention, the fluidity of the molten metal is made higher, so that no lubricant or no heat-insulating releasing agent are required. Therefore, preparation and adjustment of the casting die can be easily achieved, and casting efficiency can be made higher.
In the conventional casting method, the casting die is heated until reaching a high temperature so as to maintain the fluidity of the molten metal. The molten metal is poured into the heated casting die. The molten metal is solidified by cooling the casting die. On the other hand, in the present invention, the fluidity of the molten metal is very high, so the casting die need not be heated. Therefore, the molten metal can be solidified in a short time, the product can be quickly solidified, toughness of the product can be greater, deformation of the product, such as with sink mark, extension, can be prevented and the quality of the product can be higher. The casting die may be used at room temperature.
In conventional gravity casting, a feeding head is formed in a casting die, and the molten metal is introduced into a cavity, by its own weight, from the feeding head. In the present invention, the fluidity of the molten metal in the casting die is very high, so the capacity of the feeding head can be reduced. In a conventional die, the capacity of the feeding head is 50-60% of capacity of the die. In the present invention, since the fluidity of the molten metal can be higher, the capacity of the feeding head can be reduced to 10-20% of the capacity of the casting die. Therefore, the molten metal can be efficiently used, and the casting die can be made easily. By reducing the capacity of the feeding head, the solidification of the molten metal can be accelerated, so that a cycle time of casting can be shorter and the casting efficiency can be improved. Further, in the present invention, the product can be easily separated from the casting die, so that the product can be taken out quickly and the casting efficiency can be improved.
There are two ways of reacting the molten metal on the deoxidizing compound in the cavity of the casting die. One way comprises the steps of: making the deoxidizing compound outside of the casting die; introducing the deoxidizing compound into the cavity; and pouring the molten metal into the cavity. The other way comprises the steps of: making the deoxidizing compound in the cavity of the casting die; and pouring the molten metal into the cavity.
The deoxidizing compound is precipitated on the inner faces of the cavity so as to react the deoxidizing compound on the molten metal thereon. To effectively precipitate the deoxidizing compound on the inner faces of the cavity, a metallic gas, which is made by evaporating a metal for making the deoxidizing compound, and a reactive gas, e.g., nitrogen gas, are reacted.
The deoxidizing compound may be introduced into or made in the cavity, in which a non-oxygen atmosphere is produced, so as not to reduce the deoxidizing function of the deoxidizing compound. The non-oxygen atmosphere may be produced by decompressing the cavity, introducing an inert gas into the cavity to purge air therefrom, etc.
The method of the present invention can be properly applied to a casting, in which aluminum or aluminum alloy is used as the molten metal. In aluminum casting, a magnesium nitride compound, which is made by reacting a magnesium gas on a nitrogen gas, and the molten aluminum are reacted so as to easily deoxidize the oxide film formed on the surface of the molten aluminum. In the case of aluminum, the oxide film is apt to be formed on the surface of the molten metal. By deoxidizing the oxide film by the magnesium nitride compound, high quality products can be produced.
In aluminum casting too, there are two ways of reacting the molten metal on the deoxidizing compound in the cavity of the casting die. One way comprises the steps of: previously making the magnesium nitride compound by reacting the magnesium gas on the nitrogen gas; introducing the magnesium nitride compound into the cavity; and pouring the molten aluminum into the cavity. The other way comprises the steps of: respectively introducing the magnesium gas and the nitrogen gas into the cavity of the casting die so as to make the magnesium nitride compound; and pouring the molten aluminum into the cavity. The magnesium nitride compound, which is the deoxidizing compound, is precipitated on the inner faces of the cavity including a core, then the molten aluminum is poured therein. When the molten aluminum touches the inner faces of the cavity, on which the deoxidizing compound has been precipitated, oxygen is removed from the oxide film on the surface of the molten aluminum by the deoxidizing function of the deoxidizing compound, so that the surface of the molten aluminum can be pure aluminum.
The oxide film formed on the surface of the molten aluminum is removed by deoxidation when the molten aluminum touches the inner faces of the cavity, so that crinkles and surface defects, which are formed on the surface of the products, can be prevented. Especially, in the case of casting products having complex shapes, it was impossible to remove the surface defects. However, in the present invention, good products having no surface defects can be cast due to high wettability and a capillary phenomenon of the molten aluminum.
In the case of making the magnesium nitride compound in the cavity, firstly the magnesium gas is introduced into the cavity, then the nitrogen gas is introduced thereinto. Magnesium is heated in an inert gas, e.g., argon gas, or a deoxidizing gas, e.g., hydrogen, until the magnesium is sublimed, so that the magnesium gas is made. The magnesium gas is introduced into the cavity. The magnesium gas is introduced together with a non-oxidizing carrier gas. The pressure and amount of the carrier gas are properly adjusted. Preferably, the carrier gas is an inert gas, e.g., argon. Magnesium is sublimed at a temperature of 700-850xc2x0 C., and the magnesium gas can be easily introduced into the cavity by the carrier gas.
When the magnesium gas is introduced into the cavity, the cavity is in the non-oxygen atmosphere. To produce the non-oxygen atmosphere, the cavity is previously decompressed or purged with the nitrogen gas, etc. Oxygen in the cavity can be removed, and the magnesium gas can be uniformly introduced into the cavity.
After the magnesium gas is introduced into the cavity, the nitrogen gas is introduced into the cavity so as to make the magnesium nitride compound. The magnesium nitride compound is mainly precipitated on the inner faces of the cavity as powders.
When the nitrogen gas is introduced into the cavity, the pressure and amount of flow of the nitrogen gas are properly adjusted. To easily react the nitrogen gas on the magnesium gas, the nitrogen gas may be preheated to warm the casting die. Reaction time may be 5-90 seconds. If the reaction time is too long, the temperature of the casting die falls, so proper reaction time is 15-60 seconds.
When the nitrogen gas is introduced into the cavity to make the magnesium nitride compound, it is important to prevent the magnesium nitride compound from reacting on the casting die. The molten metal directly touches the inner faces of the cavity, so a surface condition of the molten metal highly influences a surface condition of the product. Therefore, the deoxidizing function of the magnesium nitride compound must work on the inner faces of the cavity.
The inner faces of the cavity must not react on the magnesium nitride compound. If an oxygen radical, etc., which easily reacts on the magnesium nitride compound, exists on the inner faces of the cavity, the deoxidizing function is lost before pouring the molten metal into the cavity. Therefore, it is improper to coat the inner faces of the cavity with an oxide material, e.g., lubricant, or a releasing agent. The inner faces of the cavity may be coated with a non-oxide material, e.g., graphite. Further, metal surfaces may be exposed in the inner faces of the cavity without coating with lubricant, etc., and the exposed metal surfaces may be treated with heat or nitrided. While the magnesium nitride compound exists on the inner faces of the cavity, the molten aluminum is poured into the cavity, then the magnesium nitride compound on the inner faces of the cavity reacts on the molten metal, so that oxygen is removed from the oxide film and the oxide film is deoxidized. With this reaction, the wettability of the molten aluminum is made much greater, the fluidity on the inner faces of the cavity is made higher and the capillary phenomenon is made active. Since the surface of the molten metal is made of pure aluminum, the products have a good external shape having no crinkles and no surface defects.